Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>

In Connection, add_signal_receiver adds object paths to
self._signal_recipients_by_object_path and they are not cleaned by
remove_signal_receiver.

As a result self._signal_recipients_by_object_path keeps growing indefinitely.

This seem to work for me. I know very little about dbus-python though, so I
could very well be doing it wrong.

https://bugs.freedesktop.org/show_bug.cgi?id=17551
https://bugs.freedesktop.org/attachment.cgi?id=18857

Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>

Specifically, DBusException and its subclasses no longer have the remote
traceback by default (although subclasses can turn it back on again
by setting include_traceback = True, and the various "programmer error"
subclasses of DBusException do have this set).

Hopefully this will stop people thinking it's a dbus-python or
telepathy-python bug when a D-Bus API like Telepathy deliberately raises an
error (and so dbus-python or telepathy-python is visible in the traceback).

By either appending to a list of callbacks, or subclassing and providing a
method, you can be notified when connections are added or removed.
Inspired by the DBusServer patch from Huang Peng.